There is an increasing demand for compact and lightweight resonators and filters configured by the combination of these resonators due to the rapid spread of radio devices that are typically portable phones. Generally, dielectric filters and surface acoustic wave (SAW) filters have been used. Recently, attention has been being drawn to filters using piezoelectric thin-film resonators, which have low insertion loss in the RF range, high power durability and high electrostatic resistance and are miniaturized and integrated in a monolithic form.
As an exemplary type of piezoelectric thin-film resonator, there is known an FBAR (Film Bulk Acoustic Resonator) type of resonator. This type of resonator is configured to have a multilayer structure composed of a lower electrode, a piezoelectric film and an upper electrode as main structural elements, and to have a cavity below the lower electrode in an area in which the lower electrode and the upper electrode face each other through the piezoelectric film in order to make it possible for the multilayer structure to vibrate freely.
Electron. Lett., 17 (1981), pp. 507-509 (Document 1) discloses a piezoelectric thin-film resonator of a via hole type. Japanese Examined Patent Application Publication No. 6-40611 (Document 2) discloses a cavity type of piezoelectric thin-film resonator configured to form a cavity between a lower electrode and a substrate. Japanese Unexamined Patent Application Publication No. 2004-64785 (Document 3) discloses another cavity type of piezoelectric thin-film resonator configured to form a recess in a substrate. Yet another type of piezoelectric thin-film resonator is disclosed in Japanese Unexamined Patent Application Publication Nos. 2006-128993, 2007-300430 and 2008-103798 (Documents 4, 5 and 6, respectively) in which at least a portion of the outer circumference of a piezoelectric thin film through which upper and lower electrode face each other is further in than the outer circumference of an area in which the upper and lower electrodes face each other.
The piezoelectric thin-film resonators disclosed in Documents 1 through 3 are configured to have a dielectric thin-film formed on a face that adjoins air below the lower electrode. Since the resonance frequency of the piezoelectric thin-film resonator is influenced directly by variation in thickness, many management items are needed to reduce a thickness dispersion at the time of film growth.
In Documents 4 through 6, the cross section of an end surface of the piezoelectric film after processing the piezoelectric film by etching has a slant shape, which degrades the resonance characteristic.